The invention relates to a fluid friction clutch having a driven primary disk which rotates in an oil-fillable working chamber of a housing that serves as a secondary part. A supply chamber for the oil is provided, and a recycle or pumpback system for returning the oil to the supply chamber is provided in the radially outer region of the working chamber. The recycle system includes a valve arrangement for electrically controlling the throughflow from the supply chamber to the working chamber.
A fluid friction clutch of this general type is known to one skilled in the art from DE 43 44 085 A1. In this known design, the supply chamber is provided within an insert part on the side of the clutch housing located opposite the flange connecting the clutch to the engine shaft. In the supply chamber, an electromagnet is arranged and supported relative to the clutch housing via a bearing. This electromagnet is held non-rotatably during operation and acts on a valve lever which opens a throughflow orifice to the supply chamber and a working chamber located behind a partition. In this case, the working chamber is provided in the outer region with profiling in the form of concentric annular projections which utilize corresponding annular ribs of the primary disk. In this way, the shearing forces acting between the primary disk and clutch housing can be increased, depending on the degree of filling the working chamber. In the radially outermost region, the working chamber contains a return bore which, as a rule, works in conjunction with a baffle which opens into a return duct leading back radially to the supply chamber. By means of this recycle or pumpback system, the oil is conveyed out of the working chamber and back in the supply chamber. As a result, the degree of filling in the working chamber can be determined by means of the electromagnet and the valve lever controlled by the latter. The primary disk is seated fixedly in terms of rotation on a drive shaft stub. Accordingly, the primary disk is connected directly to the engine shaft. The clutch housing is conventionally assigned fan blades which work in conjunction with a radiator for the engine coolant. The rotational speed of the fan can be controlled via the electromagnet, as a function of various parameters.
In this case, with the exception of the possibility for electric control from outside, the functioning of such a fluid friction clutch corresponds to the mode of operation of known bimetal-controlled visco fan clutches (DE 3 226 634 C1). In this case, the regulated valve controls the filling of the working chamber from the supply chamber, and the necessary pressure drop results from the centrifugal force occurring due to the rotation of the supply chamber which rotates together with the clutch housing. In such designs, the filling of the working chamber depends strongly on the rotational speed of the clutch housing, which corresponds to the fan speed. Therefore, low fan speeds, such as are provided in accordance with the requirement for a low fan idling speed (approximately 200 revolutions per minute), may last up to several minutes, until an appreciable rise in the fan speed occurs as a result of the filling of the working chamber.
In the known designs, oil is continuously conveyed out of the working chamber into the supply chamber by the baffle, and at all the stable operating points between fan idling and the fan being fully cut in, the oil quantity (regulated by the valve) flowing to the working chamber corresponds to the quantity conveyed back. In this case, the differential pressure generated by the baffle is dependent on the rotational speed difference between the driven primary disk and the clutch housing containing the fan blades. In other words, the differential pressure generated by the baffle is dependent on the slip speed. If the fan speed is lowered, the level of oil in the working chamber is reduced by means of more pronounced throttling, as a rule. At very low slip (differential) speed values (lower than 50 revolutions per minute, corresponding to full cut in at medium drive speeds), the effect of the baffle pump is minimal. As a result, several minutes may elapse until an appreciable reduction in the fan speed occurs. The dynamic behavior of the known types of fluid friction clutches is therefore characterized by dead times or lag times.
At low fan speeds, the pressure at the valve bore is very low, and only slight filling takes place. Since a differential speed is constantly present, more oil is pumped away than can flow in. This has the result that the clutch can no longer cut in.